It's long been established that Arctic Ocean sea ice
is on the retreat, writes Tim Radford. But it's the pace of change that's
surprising scientists: latest studies show that the ice-free period is
increasing by 5 days / decade.Ice in the Arctic continues
to retreat. The season without ice is getting longer by an average of five days
every 10 years.

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And in some regions of the Arctic, the autumn
freeze is now up to 11 days later every decade.

The findings come in a new study in Geophysical Research
Letters - 'Changes in Arctic melt season and implications
for sea ice loss' by Stroeve et al.

That means that a greater proportion of the polar
region for a longer timespan no longer reflects sunlight but absorbs it. This
change in albedo - the scientist's term for a planet's reflectivity - means
that open sea absorbs radiation, stays warmer, and freezes again ever later.

Warming accelerates

None of this is news: sea ice in the Arctic has
been both retreating and thinning in volume for four decades.

Researchers have tracked the retreat of the snow
line to find tiny plants exposed that had been frozen over 40,000 years ago:
the implication is that the Arctic is warmer now than it has been for 40
millennia.

This warming threatens the animals that depend
for their existence on a stable cycle of seasons and is accelerating at such a
rate that the polar ocean could be entirely free of ice in late summer in the
next four decades.

So Julienne Stroeve, of University College London
and her colleagues have provided yet further confirmation of an increasing rate
of change in the region in their latest study.

The scientists examined satellite imagery of the
Arctic for the last 30 years, on 25 square kilometer grid; to work out the
albedo of each square for every month they had data.

Their headline figure of five days is an average:
in fact the pattern of freeze and thaw in the Arctic varies. In one region the
melt season has been extended by 13 days, in another the melt season is
actually getting shorter.

Energy increases

This increasing exposure to summer sunlight means
that ever greater quantities of energy are being absorbed.

"The extent of sea ice in the Arctic has
been declining for the last four decades",
said Professor Stroeve. "And the timing of when melt begins and
ends has a large impact on the amount of ice lost each summer."